The normal diet of the ruminant animal is forage. Forage includes grasses, legumes and cellulytic byproducts of agricultural production. These are either fed fresh as pasture or green chop; in a dry form as hay; or in a preserved state as silage. The ability to utilize these materials as sources of nutrients is only possible as a result of pregastric bacterial fermentation in the rumen, the nonfundic portion of the animal's stomach. Here, bacterial action reduces the complex structural carbohydrates; cellulose, hemicellulose, and lignin and the associated nonstructural carbohydrates; pectin, starches and sugars, to either fatty acids or more chemically simplistic carbohydrate forms, which are then subjected to gastric action in the fundic stomach and small intestine.
The adaptation of ruminants to pregastric digestion has involved a system of retention of digesta, which is an essential part of the mechanism for maximal extraction of energy. This retention requires some sacrifices in food intake, which becomes more limited on forage based diets because the coarser ingesta must be retained longer to achieve efficient extraction of energy. This poses a special problem in the modern, domesticated ruminant, in that the nutrient demands created by genetic selection for rapid lean muscle growth or high levels of milk production far exceed the supply generated by rumenal fermentation of forage based diets. The diets that must be fed require the addition of large amounts of nonstructural carbohydrate (starches and sugars) fed in the form of grain which, unfortunately, often is a source of physiologic and metabolic stress. These problems are associated with the changes which occur in rumenal fermentation as a result of grain ingestion. As a consequence, feeding strategies must attempt to maximize forage use while not compromising nutrient supply needed for maintenance and production.
A solution to the problem of nutrient supply and demand in the ruminant animal, as imposed by the limitations of bacterial, pregastric digestion, is to enhance the efficiency and rate at which this process occurs. The rumen is a continuous fermentation system that is provided with nutrients (feeds), buffers (salivary and other salts) and fluids (water and saliva) on both a continuous and an intermittent basis. The efficiency of this fermentation is measured through rumen turnover. Turnover is conventionally expressed as the portion of the rumen contents that leaves the rumen per hour. Liquids and solids turn over at different, but usually related, rates. Liquid flow rates, as proportions of the total liquid volume, have been found to turn over at rates that increased from &lt;8 to 13.5%/hr as dry matter intake went from 5 to 21 kg/day (Livestock Prod. Sci., 17:37, 1987). At the same time, solids turnover increased from 3 to 5%/hr due to increased intake. In other studies, values of 17%/hr for liquids (Can. J. Ani. Sci., 64 (Supp.):80, 1984) and as high as 7.0%/hr for concentrates (J. Dairy Sci., 65:1445, 1982) were reported. In a typical ration of a dairy cow consuming &gt;20 kg dry matter/day, representative rumen digesta passage rates would be 15%/hr for liquids, 6%/hr for grains and 4.5%/hr for forages. The rates would all decrease with a lower level of intake.
Another important rumen characteristic associated with turnover rate is microbial yield, where microbial yield is defined as the quantity of microbial mass flowing from the rumen per day. A further, and important refinement of this expression of microbial yield, which is also effected by turnover rate, is the efficiency of microbial yield. This is usually expressed as grams of microbial protein (or nitrogen) produced per kg of organic matter (OM) digested in the rumen. Both aspects of microbial production have applied significance. Microbial yield is important as an index of the amount of microbial protein available to the ruminant animal per day. Microbial efficiency is important as part of the calculation of microbial yield where: microbial yield (gr of microbial N/day)=microbial efficiency (gr microbial N/kg digested organic matter).times.kg OM digested in the rumen per day.
Because of the rapid rumen turnover rates commonly found in cattle with high dry matter intakes, such as dairy cattle, high microbial efficiencies are expected. If, however, an imbalance in the nutrients available to the rumen microbes occurs, the microbial efficiency can be impaired. This is particularly evident if ruminally available nitrogen or carbohydrate sources are inadequate.
Rumen microbes, with a few exceptions, use only carbohydrates (CHO) for energy, and growth will be proportional to the amount of carbohydrate fermented. This relationship is expressed by the equation: ##EQU1##
In this equation, carbohydrate digested includes the fermentable portion of the fibre, plus sugars and starches. In practice, the analytical techniques used to determine carbohydrates do not clearly delineate the contribution made to the various sources in a forage-grain ration. Commonly, neutral detergent fiber (NDF) is used to quantitate the total structural or cell wall carbohydrates, which include cellulose, hemicellulose and lignin. Sugars and starches are not individually determined, but are included, along with pectins, gums and other components, in a fraction referred to as non-structural carbohydrate (NSC). As the digestibility of NSC in the rumen is considerably higher than that of NDF, it follows that the amount of total carbohydrate digested per day is positively related to the proportion of NSC in the diet. The primary source of NSC in the diet of dairy cows is grain. However, as previously indicated, it is both nutritionally and physiologically desirable to obtain a greater portion of the ruminally available carbohydrate from the forage portion of the diet, so that the risk associated with feeding high levels of grain to the animal is reduced.
Although energy, i.e., carbohydrates, is usually considered to be the most limiting nutrient for maximum microbial growth, in normal diets currently fed to domestic ruminants, ruminally available nitrogen, derived from the protein component of the ration, is often more limiting than available energy. Experiments (J. Dairy Sci., 65:1445, 1982) have shown that while increasing ruminally degradable nitrogen had a small effect on carbohydrate digestion, it had a marked effect on microbial efficiency. Ruminal bacteria will effectively utilize several sources of degradable nitrogen. Most species can and will use ammonia for growth. However, all species prefer amino acids or peptides, with larger peptides being taken up in preference to small peptides and amino acids. Studies in vivo and in vitro, both using labeled nitrogen, support this concept. However, maximal microbial efficiency is best supported by providing adequate amounts of nitrogen in forms that synchronize with the degradation of ruminally available carbohydrates. In terms of the nitrogen input, this is most easily achieved by combining nonprotein nitrogen, peptides and amino acids with varying solubilities, thus achieving the equivalent of a nitrogen steady state within the rumen. Conventional feed stuffs however, have not been shown to provide any of these elements in sufficient balance to achieve steady state. Release rates of peptides and amino acids from feedstuffs for example, are dictated by microbial degradation of intact protein. Rather than be stimulatory to microbial growth and efficiency, their availability is a function of it. The usual sources of nonprotein nitrogen are inorganic materials such ammonium salts or urea. These materials are so soluble that they volatilize rapidly within the rumen almost upon ingestion and a large portion of their nitrogen is lost as ammonia before rumenal bacteria can effectively utilize it. They are also unpalatable and toxic. Thus, the formulating of ruminant diets with the objective of achieving a nitrogen steady state in the rumen has remained elusive.
During certain phases of the production cycle of domestic ruminants, nutrient intake can be compromised by a number of physiologic factors. For example, in the period immediately preceding parturition, changes in hormonal balance associated with that event can negatively effect gastrointestinal motility, such that feed intake is reduced. The resulting reduction in nutrient availability to the animal has been associated with a number of metabolic disorders which are common to domestic ruminants during this period. An example is the metabolic disease, "milk fever" caused by a state of systemic hypocalcemia which results from parturient inappetence. One avenue of research that has been proven to offer some relief from this problem is the dietary manipulation of ionic balance in the prepartum ration. Diets normally fed ruminant animals are highly cationic in content. This is reflected in blood pHs that tend to be basic and in the animal's highly basic urine pH, usually in the range of pH 8 or higher. By shifting the dietary balance of cations and anions in favor of anions for a period of time prepartum, a metabolic acidotic state can be achieved, and blood pH is reduced. Urine pH, the most easily measured response to a shift to an anionic diet is reduced and the degree of acidity is a function of the success of this shift. It is widely recognized by practitioners in the art that efficacy of dietary ionic shift is reflected in urinary pH reduction. Values below pH 8.0 are acceptable but it is preferred that urine pH values fall below neutrality or pH 7.0. To accomplish this, it is recommended that diets have a cation/anion balance that is as negative as is possible, usually below (-)60 meq/kg. This shift in homeostasis has been shown to increase feed intake, and improve nutrient homeostasis, thereby reducing the incidence of the resulting, associated metabolic and physiologic disorders such as milk fever.
Currently dietary ionic shift has been attempted through the supplementation of combinations of anionic salts. Examples include ammonium chloride, ammonium sulfate, calcium chloride and calcium sulfate. However, anionic salts as a group are highly unpalatable to ruminant animals and potentially toxic due to their extreme solubility of the nitrogen component. Dry matter intakes where anionic salts have been fed are routinely reduced to a point where nutrient balance as a whole is compromised. Consequently, although the concept is widely accepted as physiologically efficacious, its practice is limited by a lack of an appropriate vehicle to achieve shift. This vehicle should be palatable, safe and metabolically effective, as indicated by blood or urine pH reduction.
Finally, another method of enhancing feed intake is to formulate diets that ensure that rumenal microbial fermentation is occurring efficiently. This is most easily achieved through synchronizing the availability of ruminally degradable nitrogen with ruminally available carbohydrate.